Light diffusing sheet is used in luminous signs, lighting covers, rear-projection screens, and other applications requiring a high level of light transmission while blurring the point light source. Effective light diffusing sheet should have excellent light transmission, good light diffusion, light scattering and weatherability.
The diffusing mechanism of a light-diffusing sheet can be caused by many different means. These include foaming the polymer (US 200200823), entrapment of air bubbles (JP 2002090515), and various surface configurations (toothed—JP202196110, concavo-convex—U.S. Pat. No. 6,741,307).
The diffusing mechanism can also be in the form of dispersed particles. The particles could be polymeric such as described in U.S. Pat. No. 6,878,436 where the polymers differ from the matrix in refractive index by at least 0.001. The particles could also be inorganic, such as BaSO4, SiO2, CaCO3, Al2O3, TiO2 and ZnO, as described in U.S. Pat. No. 4,418,986 and U.S. Pat. No. 6,875,499.
Surprisingly it has been found that the combination of both selected polymer particles and selected inorganic particles in a thermoplastic matrix produces a synergistic effect, leading to an increased luminous transmission with good diffusion, making the light-diffusing sheet visually attractive.